Try to minimize the tag browser to the left. I have have it minimized all the time, except when I actually need it.

Usually when you update metadata only some records in the database is changed. But when you update authors or title, then the books have to be moved to a new folder in the calibre library.

Also when the books are moved they are really first copied to the new place and then deleted at the old. Not just renamed. The reason is that this reduces the chance of data corruption if the process is interrupted for some reason. If only one book is updated, then the disc caches can most likely be fully used to speed it up. But maybe not as likely if several books are updated.